Magnetic field assisted synthesis of magnetic nanoparticles

ABSTRACT

A process for the production of magnetic nanoparticles having uniform size and shape in which a magnetic metallic substance selected from magnetic transitional metals, their alloys, and intermetallic alloys with non-magnetic metals, is dissolved in a non-aqueous medium, reacted with a reducing agent, in solution and the reaction mixture is subjected to an external magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/60,347 filed Sep. 1, 2004, the content of which is incorporatedherein by reference in its entirety.

GOVERNMENT LICENSE RIGHTS

The United States Government has rights in this invention as providedfor by U.S. Defense Advanced Program Agency (DARPA) Grant No.F33615-01-2-2166.

BACKGROUND OF THE INVENTION

Magnetic nanoparticles are known to be very important in a wide varietyof applications, such as ultrahigh density magnetic data storage,catalysts, drug delivery, cell separation, diagnostic, hypothermia forcancer treatment, inductive bonding, ferrofluids and magnetic sensors.If such magnetic nanoparticles are to be used with the greatesteffectiveness, it is crucial that they be produced with uniform size andshape and in a process that can be easily scaled up for large quantityproduction.

Existing techniques for the production of magnetic nanoparticles areproblematic in that they are very costly, impractical for massproduction or involve the use of undesirable toxic materials. Forexample, monodispersed nanoparticles of transition metals and theiralloys may be produced by a polyol process, but it requires the use ofexpensive noble metals as seeds in order to obtain the desired smallsize.

Micro-emulsion or reverse micromulsion methods have produced acceptablemagnetic nanocrystals but they are unsuitable for large quantityproduction.

Thermal decomposition methods are also known to produce magneticnanoparticles but the organic metallic compounds used in this process,such as iron pentacarbonyl [Fe(CO)s], dicobalt octacarbonyl [CO₂(C)₈],nickel tetracarbonyl [Ni(CO)₄], are expensive, unstable and very toxic.Thus, such methods are not conducive for large quantity production.

This invention avoids these problems and produces large quantities ofuniformly-sized magnetic nanoparticles by controlling the growth of theemerging nanoparticles as they are formed through the use of an externalmagnetic field.

SUMMARY

In this invention, a novel process is provided for producingnanocrystals of uniform shape and size from magnetic transition metals,salts thereof, alloys of said metals and alloys of said metals withnon-magnetic metals.

The process comprises essentially dissolving the selected magnetictransition metal, salt or alloy thereof in a non-aqueous solution andreducing the dissolved transition metal, salt or alloy in the presenceof an external magnetic field.

In a preferred embodiment, borohydrides are employed as the reducingagent and the transition metal or alloy is dissolved in ethanol.

By performing the reduction reaction in a non-aqueous medium one is ableto avoid the formation of transition metal borides which have muchweaker ferromagnetism than pure transition metals and their alloys. Byemploying an external magnetic field in the reaction solution, one isable to control the nucleation, growth and size of the magneticnanoparticles so as to achieve very uniform nanoparticles in largequantity. Any non-magnetic particles in the solution will not beattracted by the external magnetic field and may be separated out.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the experimental setup utilized in theexample described herein.

FIG. 2 presents TEM graphs (a) and (b) show size distribution and shapeand (c) to demonstrate selected area electron diffraction pattern.

FIG. 3 shows an XRD pattern of Co nanocrystals as-synthesized (red line)and subsequently annealed at 600° C. in Ar atmosphere for 1 hour (blueline). Vertical bars indicate the diffraction peak positions of fcc Cofrom JCPDS number 15-0806.

FIG. 4 is a hysteresis loop of as-synthesized Co nanocrystals (a, blueline) and subsequently annealed at 600° C. (b, blue line) for 1 hour andat 700° C. (b, red line) for 3 hours in H2 atmosphere.

DETAILED DESCRIPTION

The magnetic nanoparticles produced according to the process of thisinvention are formed from magnetic transition metals such as iron,cobalt and nickel, salts thereof, their alloys and intermetalliccompounds such as Co₃Pt, CoPt, CoPt₃, Fe3Pt, FePt and FePt₃.

It is important to this invention that the metals or their alloys whichform the intended magnetic nanoparticles are reduced in a non-aqueoussolution, preferably a lower alcohol such as methanol, ethanol,propanol, isopropanol and butanol. Ethanol is especially preferred.

Any of the known reducing agents may be used in the process of thisinvention, although a strong borohydride such as NaBH₄ or KBH₄ ispreferred.

It is crucial to the formation of magnetic nanoparticles of uniform sizeand shape that the reaction solution be subjected to an externalgradient magnetic field so as to adjust the dwelling time of thenewly-formed magnetic nanoparticles in the reaction site, thuscontrolling their growth.

The materials produced by this invention may be used in exchangedcoupled soft and hard magnets, ultrahigh density data storage media,catalysts, biological and medical applications such as cell separation,targeted drug delivery and hyperthermia treatment of tumors andinfectious diseases.

EXAMPLE

CoCl₂ and NaBH₄ ethanol solutions were first prepared by dissolvingappropriate amount of CoCl₂ and NaBH₄ into ethanol with vigorousstirring. The molar ration of NaBH₄ over CoCl₂ was purposely made largerthan 2 to ensure complete reduction of CoCl_(2.) The CoCl₂ ethanolsolution was then put on top of a mineral oil layer which has a densitylarger than the above solution. NaBH₄ ethanol solution was added intoCoCl₂ ethanol solution in a drop-like manner by using a dropping funnelor better using an ultrasonic atomizer that can give rise to smalldroplets of ˜40 μm. According to Jackelen et al. [A. M. L. Jackelen, MJungbauer, G. N. Glavee, Langmuir 1999, Vol. 15, PP 2322-2326.], thefollowing chemical reactions take place instantaneously when the NaBH₄ethanol solution is added:CoCl₂+2NaBH₄→Co+H₂+B₂H₆+2NaClB₂H₆+6CH₃CH₂OH→2B(OCH₂CH₃)₃+6H₂

The magnetic Co particles were attracted by a permanent magnet placedunder the reaction beaker, which create a gradient magnetic field, (seeFIG. 1) first into the interface between the oil and ethanol phases andfinally into the oil phase. In this way, the particles are immediatelytaken away from the reaction site where BH₄ ⁻ ions are continuallysupplied. After the reaction was completed, the supernatant solution andthe oil were discarded. The slurries were washed by ethanol, acetone andde-ionized water to thoroughly remove the residual oil and NaCl formedduring the reaction, and finally washed by acetone again to removewater.

The resulting nanoparticles are shown in the drawing to have uniformsize and shape. FIGS. 2 a and b show the TEM graphs of theas-synthesized Co nanocrystals and confirm such a result. Veryhomogeneous particles with near perfect spherical shape are clearly seenin FIG. 2 b, a histogram made using 250 randomly selected particlesshows that the average particle size is around 4.7 nm with standarddeviation of 1.6 nm. Since the as-prepared particles are small, the XRDpattern shows only a very broad peak [FIG. 3, bottom trace]corresponding to the diffraction line of face centered cubic (fcc) Co.After annealing the as-prepared particles at 600° C. for 1 h with argonflow, all diffraction lines of fcc Co appear but no Co—B alloydiffraction lines are detected [FIG. 3, upper trace]. In contrast to thepresent invention, this is usually found when the reduction is carriedout in an aqueous solution. Therefore, by the method described here, onecan fabricate pure Co nanoparticles. Although the XRD pattern for theas-prepared μCo nanoparticles lacks fine structure, the crystallinitywas evidenced by the TEM associated electron diffraction pattern [FIG.2(c)] indicating that the particles are well-crystallized fcc Co.

In FIG. 4(a), the M-H loop (solid triangles and line) for theas-prepared Co nanoparticles is plotted and measured at RT, showingtypical superparamagnetic behavior. The magnetization curve can befitted to a Langevin function with the summation of two size components[FIG. 4(a), dotted line; fitting parameters are indicated on the figure]M=M₁×[coth(μ₁H/k _(B)T)−k _(B)T/μ₁H]+M₂×[coth(μ₂H/k _(B)T)−k _(B)T/μ₂H]where M_(i) and μ_(i) (i=1, 2) are the saturation moment and theeffective moments of a unit magnetic cell of each size component,respective and k_(B) is the Boltzmann constant.

The TEMP graph shown in FIG. 2(b) indicates that the particles arealmost perfectly spherical.

The zero-field cooled (solid triangles and dotted line) and 5 T-fieldcooled (solid circles and line) M-H loops (FIG. 4(b)′ measured at 5 Knearly repeat each other, implying that the oxidation of particles wereminimized.

While there is shown and described certain specific structures embodyingthe invention, it will be manifest to those skilled in the art thatvarious modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described.

1. A process for producing magnetic nanoparticles of uniform shape andsize comprising: a. forming a solution in a non-aqueous medium of amagnetic substance selected from the group consisting of magnetictransition metals, their salts and alloys and intermetallic alloys ofsaid magnetic transition metals with non-magnetic metals; b. adding areducing agent to the solution to form a reaction mixture, and c.applying an external gradient magnetic field to the reaction mixture. 2.A processing according to claim 1 in which the non-aqueous medium is alower alcohol.
 3. A process according to claim 2 in which the alcohol isethanol.
 4. A process according to claim 1 in which the reducing agentis a borohydride.
 5. A process according to claim 3 in which themagnetic substance is cobalt.
 6. A process according to claim 1 in whichthe magnetic substance is cobalt.
 7. A process according to claim 1 inwhich the magnetic substance is a cobalt alloy with a non-magneticmetal.
 8. A process according to claim 1 wherein the magnetic substanceis an alloy of cobalt and another magnetic transition metal.
 9. Aprocess for producing magnetic nanoparticles of uniform size and shapecomprising: a. dissolving a cobalt salt in ethanol; b. dissolving aborohydride compound in ethanol; c. adding the borohydride solution tothe cobalt salt solution so as to cause a reduction reaction in themixture; d. applying an external magnetic field to the reaction mixture.